fix parsing outzip in afhq data preprocessing

[mshahbazi72]

outzip is defined as a positional argument (no -- option before the name) in dataset_preprocessing/afhq/runme.py, line 22. Therefore, the use of the argument "required=False" in line 22 throws the following error: TypeError: 'required' is an invalid argument for positionals

This pull request replaces "outzip" with "--outzip" in dataset_preprocessing/afhq/runme.py, line 22m to fix the following issue.